the quality of water that causes it to impair the lathering of soap: caused by the presence of certain calcium salts. Temporary hardness can be removed by boiling whereas permanent hardness cannot
Word Origin and History for hardness
n.
Old English heardnysse; see hard + -ness. Meaning "difficulty of action or accomplishment" is late 14c.
hardness in Science
hardness
(härd'nĭs) A measure of how easily a mineral can be scratched. Hardness is measured on the Mohs scale.